Conventionally, as a low-heat-loss stabilized power supply which operates with comparatively high efficiency in a large input-output difference condition, switching regulators have been widely used which generate a desired output voltage from an input voltage by driving an energy storage device (a capacitor, an inductor, or the like) by performing on/off control (duty control) of an output transistor.
For switching regulators required to provide high regulating efficiency, a configuration has been adopted in which, for the purpose of minimizing the on-resistance of a rectifier device, a synchronous-rectification transistor is employed as the rectifier device and is on/off-controlled complementarily to the output transistor.
One disadvantage of the above described synchronous-rectification switching regulator is that, when its switching is controlled uniformly regardless of load variation, the amount of power lost therein is larger in a light load condition. Such an increase in power loss occurs because a lighter load permits an inductor to store a smaller amount of energy so that the energy stored in the inductor is all discharged in a period shorter than the on-period of the synchronous-rectification transistor (a period during which the synchronous-rectification transistor should be on, that is, the stored energy discharging period) and thereafter until the on-period of the synchronous-rectification transistor ends, a reverse current flows from an output terminal.
Conventionally, as a solution to the above described problem, synchronous-rectification switching regulators have been disclosed and suggested (see Patent Publication 1) which are provided with a reverse current detecting circuit for detecting a reverse current flowing toward a voltage input terminal from an intermediate connection node between an output transistor and a synchronous-rectification transistor, and when a reverse current is detected in an on-period of the synchronous-rectification transistor, the synchronous-rectification transistor is not allowed to be on at least when the next period comes during which the synchronous-rectification transistor is to be on.
Other conventional arts related to the present invention have been disclosed and proposed as exemplified by: a power supply circuit in which the on-resistances of an output transistor and of a synchronous-rectification transistor are used to detect a load current during each on-period, and when the load is found to be light based on the detection result, the synchronous-rectification transistor is turned off; and a voltage changing circuit that detects a load current according to the potential difference between the source and the drain of an output transistor right after the output transistor is turned on, and based on the detection result, switches between a PWM (pulse width modulation) control and a PFM (pulse frequency modulation) control (see Patent Publications 2 and 3).
Patent Publication 1: JP-A-2002-281743
Patent Publication 2: JP-A-H11-146637
Patent Publication 3: JP-B-3511195